The disclosure relates to a hydrostatic drive.
Such hydrostatic drives are used, for example, in the case of commercial vehicles as an additional drive for the front wheels, while the rear axle is driven via a conventional mechanical drivetrain.
Such a commercial vehicle with conventional and hydraulic drivetrain is explained, for example, in DE 42 12 983 C2. In the case of this solution, the hydraulic drivetrain can, where necessary, be connected via a valve arrangement, wherein each wheel of the front axle is assigned a hydraulic motor which is supplied with pressurizing medium via a variable displacement pump which can be pivoted across zero in order to drive the front wheels. The known system is furthermore embodied with a retarder via which the brake system of the commercial vehicle are hydraulically assisted during braking.
One disadvantage of this solution is that both in the case of the retarder function and in the drive function, the pressurizing medium is heated to a significant extent in the hydraulic drivetrain which forms a closed circuit. A further disadvantage of this solution lies in the fact that, in the case of a disconnected hydraulic drivetrain, the hydraulic motors run along with the wheels of the rear axle and thus increase the fuel consumption as a result of the friction inherent therein.
In order to avoid the first disadvantage mentioned above, DE 39 26 354 C2 describes a hydraulic motor which is embodied with a flushing valve in order to remove a proportion of the pressurizing medium from the closed circuit and feed it back to a tank from which the removed quantity is balanced out via a feed pump. In this manner, excessive heating of the pressurizing medium can be reliably avoided.
In order to avoid the second disadvantage mentioned above, a vehicle with a conventional mechanical drivetrain and a hydraulic drivetrain is explained in U.S. Pat. No. 6,367,572 B1. Here, a hydraulic motor is assigned to both wheels of the front axle and is of a radial piston design. Such a radial piston machine has a plurality of pistons supported on a lifting ring, which plurality delimits a working chamber, wherein the working chambers are consecutively connected to high pressure and low pressure in order to drive the hydraulic motor. When disconnecting the hydraulic drivetrain, it is moved into a “freewheel mode” in the case of which tank pressure or a comparatively low pressure acts on the working chambers, while a feed pressure or another pressure which is greater than the tank pressure acts on the lifting ring—or on the housing side. The differential pressure which results from the higher housing pressure brings about a “retraction” of the pistons so that they lift off from the lifting ring and thus the friction is reduced in the case of a disconnected hydraulic drivetrain. In the case of adjustment of the pressure difference via the piston, it must be ensured that the pressure difference which is active in the lifting-off direction is so large that it holds the pistons in the lifting-off position counter to the centrifugal forces which are active during rotation.
It is disadvantageous in the case of this solution on the one hand that a comparatively high pressure is active in the housing if the hydraulic drivetrain is moved into the freewheel mode. Moreover, in the case of this known solution, the pressurizing medium can heat up since in turn no housing flushing is provided.
Against this background, the object of the disclosure is to create a hydrostatic drive which enables operation in the “freewheel mode” with minimal effort and in the case of which heating of pressurizing medium can be largely avoided.
This object is achieved by a hydrostatic drive with the features of the disclosure.
Advantageous further developments of the disclosure are the subject matter of the subordinate claims.